EPR study of the free radicals in the single crystals of 2-thiothymine γ-irradiated at 300 K

EPR spectroscopy has been used for the study of the free radicals formed in the single crystals of 2-thiothymine (5-methyl-2-thiouracil) γ-irradiated at room temperature (300 K). The radicals formed by a net hydrogen-atom addition to C(6) (5-thymyl or 5-yl radicals) and by a net hydrogen abstraction from the methyl group (7-thymyl or 7-yl radicals) has been studied in detail. The hyperfine tensors of proton couplings in these radicals are given together with their g-tensors. It has been shown that the calculated hyperfine proton couplings and relative orientation of the coupling tensors along the pyrimidine ring are similar to those observed earlier in 5-yl and 7-yl radicals in the single crystals of thymine and its derivatives. Although no large differences in hyperfine couplings were found, calculated values of the g-tensor elements of the 5-yl radical are appreciable larger, what is expected due to the presence of sulfur at C(2) in 2-thiothymine. Also, the observed superhyperfine 1:2:2:1 splitting of the resonance lines of the 5-yl radical indicated that the presence of sulfur in 2-thiothymine slightly affects the spin distribution in this radical insomuch that appreciable amount of spin is located on one of N–H fragments of 2-thiothymine.